The immune system of teleosts (and specifically, the rainbow trout) is extraordinarily interesting from an immunological point of view, as these animals use the kidney as their major immune organ. The anterior kidney of trout appears to be functionally similar to mammalian bone marrow. This site is dedicated entirely to hematopoiesis, including all B cell development, while it also maintains a population of antibody-secreting plasma cells. In contrast, the posterior kidney harbors populations of mature and activated B cells, intertwined with blood filtering nephrons. Hence, the immune system of trout is quite unique in the sense that the kidney, traditionally known as a blood-filtering organ, is also the main site for B cell development and possibly, B cell activation. We chose to use trout as a model for teleost species because zebrafish (considered the main animal model for the teleost), is too small to provide enough cellular material to perform the proposed experiments. With the limited information available on cell surface differentiation markers for trout immune cells, and without the corresponding serological reagents, we have developed an alternative approach for studying B cell differentiation. This approach takes advantage of the highly conserved nature of DNA binding domains of developmentally expressed, lymphoid-specific transcription factors. Fixed cells will be stained with Pax-5 and Blimp-1 specific antibodies, in combination with trout-specific anti-immunoglobulin antibodies, using fluorescent microscopy and flow cytometry. Both non-immunized and immunized trout will be analyzed. This will enable us to explore the relative abundance, location, and changes of unique B cell subsets in the trout kidney during an immune response. An additional aim involves the isolation of trout Pax- 5 and Blimp-1 genes with the goal to develop monoclonal antibodies as well as to set the stage for long-term exploration of the function of these two factors in trout humoral immune responses. The studies aim to increase understanding of the trout humoral immune system and are expected to reveal important similarities between the trout and mammalian immune systems, as well as uncover trout-specific structures and functions that are either absent in mammals or have adapted in distinct ways. Results should provide new insights in the diverse and unique usage of immune tissues in vertebrate animals, yet with the shared goal of a highly effective immune response. As such, this study should add important knowledge in the fields of comparative and evolutionary immunology, as well as areas of autoimmune disease, including lupus and inflammatory kidney disease. [unreadable] [unreadable] [unreadable]